The subject invention relates to projection television systems, and more particularly to correction circuits for a projection television of the type having three picture tubes arranged side by side on a horizontal axis.
In a projection television system, the image generated on the face plate of the picture tube is magnified through a lens and projected on to a screen. In a color projection television system, there are three picture tubes which separately display red, green and blue information of a color television picture. These three picture tubes may be arranged in a triad or side by side on a horizontal axis. In the system for which the correction circuits according to the subject invention are designed, the three picture tubes are arranged side by side on a horizontal axis as shown in FIG. 1 of the drawings.
In any color projection television, the images of all three picture tubes must be superimposed so that a single image appears on the screen. Each image must be exactly the same size and shape so that the superpositions can be accomplished. In the system shown in FIG. 1, the center picture tube is projected perpendicular to the screen while the two outer tubes are projected at an angle such that superposition is possible. This angular projection causes an optical distortion known as trapezoidal distortion. FIG. 3 is a grid illustrating trapezoidal distortion. As shown in FIG. 3, the vertical left edge is shorter than the vertical right edge. This distortion would be produced from the left picture tube as shown in FIG. 1. FIG. 2 illustrates the uncorrected images projected on to the screen by the center and right picture tubes as illustrated in FIG. 1. The image projected from the left picture tube in FIG. 1 is omitted from FIG. 2 for purposes of clarity. As shown in FIG. 2, the image projected from the center picture tube is perfectly rectangular, while the image projected from the right picture tube suffers from trapezoidal distortion. In addition to the trapezoidal distortion, there is a further optical distortion along the horizontal axis as illustrated in FIG. 2.
Besides the distortions produced by the projection angles of the left and right picture tubes, the uncorrected television image also suffers from a distortion known as pin cushion distortion. FIG. 4 is a grid illustrating pin cushion distortion. This distortion may be characterized as a general concavity of the top and bottom and side edges of the image. In the past, top and bottom pin cushion correction has been accomplished in the main deflection yoke by attempting to modulate and superimpose a horizontal parabola on the vertical sweep current wave form. Side pin cushion correction is usually done by modulating the horizontal yoke current with a vertical parabola. This approach for correctng pin cushion distortion has been inefficient and somewhat lacking in performance. The screen is much larger in a projection television system, and therefore the distortions appear larger and more noticeable.